(CBS News) WASHINGTON -- A congressional investigation viewed video Thursday that was recorded by security cameras the night of the attack on the U.S. consulate in Benghazi, Libya.

The first video of the attack was captured by cell phones. But it took about 10 more days for the FBI to get its hands on video taken by security cameras at the consulate. That classified video, shown to the House and Senate Intelligence Committees, answers some of the many questions surrounding the attack.

There has been a lot of controversy about whether the Sept. 11 attack -- which killed four Americans, including the U.S. ambassador -- was planned by terrorists or the result of a mob angered by an internet movie ridiculing Islam.

A U.S. intelligence official said the first video shows no sign the assault arose -- as first thought -- out of a demonstration. But it also indicates the attack, though intentional, was not well planned.

The video makes clear the attackers did not know the layout of the compound. The attack lasted about an hour and ended with the consulate in flames, a scene captured by a surveillance drone overhead.

The surviving Americans retreated to a CIA safe house about a mile away, where they continued to take fire from small arms and rocket-propelled grenades.

After another hour, the shooting stopped. For four hours, no shots were fired, giving a rescue team flown in from Tripoli time to make its way to the annex.

Ten minutes after the rescuers arrived, a second attack began, this time with mortars. That attack was also captured on video -- both by security cameras and a drone overhead -- and it shows a more organized and deliberate assault than the one on the consulate.

Intercepted cell phone calls and text messages from that night indicate that some of those who attacked the consulate were inspired in part by televised scenes of protesters storming the American embassy in Cairo, enraged over an anti-Muslim video running on the internet. Others seemed to have nothing more in mind than looting.

That first attack created an opportunity for the second attack -- the one on the annex. Whether it was planned that way all along is not clear.

U.S. officials say the four-hour lull in the fighting gave the attackers time to bring in more firepower in the form of mortars. They fired five rounds: the first two missed, while the next three hit.